This invention relates to beamformers, and in particular to a transmit beamformer that provides improved focusing.
Ultrasonic imaging is widely used in many settings, including medical applications. A typical ultrasonic imaging system includes an array of transducers, a transmit beamformer, and a receive beamformer. The transmit beamformer supplies transmit waveforms (which may be voltage waveforms) to the transducers, which in turn produce respective ultrasonic transducer waveforms (which are pressure waveforms). In a phased array system, the transmit waveforms are delayed in time to cause the ultrasonic waveforms to interfere coherently in a selected region in front of the transducers.
Structures in front of the transducers scatter ultrasonic energy back to the transducers, which generate associated receive waveforms (which may be voltage waveforms). These receive waveforms are delayed for selected times that are specific for each transducer such that ultrasonic energy scattered from a selected region adds coherently, while ultrasonic energy from other regions does not.
It is well recognized that the absorption characteristics of the body being imaged can have a significant impact on the operation of an ultrasonic imaging system. For example, the ultrasonic absorption coefficient of living tissue increases with frequency, and lower frequencies are therefore preferred for imaging at greater depths. Higher frequencies provide improved resolution in the range dimension than lower frequencies, and higher frequencies are preferred for imaging at shallower depths.
Pittaro U.S. Pat. No. 5,113,706 discloses an ultrasonic imaging system that divides the body being imaged into several zones, and uses a separate burst of ultrasonic energy at a separate frequency and power level for each zone. In this system, transmit focus and power are stepped over the entire multi-zone focal range of interest, with successive bursts that increase in focal depth, decrease in frequency, and increase in power.
One disadvantage of the system disclosed in the Pittaro patent is that multiple bursts are fired for each transducer steering position. Such multiple bursts can increase the time needed to complete an entire image. The Pittaro patent makes a brief suggestion at column 13, lines 35-39 that frequency multiplexing can be used so that the multiple wavefronts for a given steering position can be concurrent rather than successive, but no further details are given.